Posted by: coastlinesproject | May 29, 2015

Texas Floods

NewsALERT™
AIR WORLDWIDE

Texas and Oklahoma Hammered by Rain and Flooding
Location: Texas and southern Oklahoma
Maximum sustained winds: 65 mph
Precipitation levels: More than 4 inches per hour
Disruptions to travel and power: Highways closed, METRO service suspended, travel discouraged
Reported damage: Homes washed away or flooded, buildings damaged, trees and utility poles downed, power outages
Forecast: More storms expected
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This was already the wettest May on record for many parts of the southern Plains states (ending five years of extreme drought in some areas), when a storm system delivered torrential downpours Monday afternoon into early Tuesday morning and sparked severe flash flooding, hail, and tornadoes. Hundreds of homes have been flooded or destroyed, eight people have been reported dead, and at least a dozen more are missing.

Meteorological Summary
The slow-moving, upper-level, low-pressure system traveled across Texas, producing thunderstorms and heavy rains in the western Houston metro area. Within a few hours Monday night, 6 to 8 inches of rain had fallen across the area from these mesoscale convective systems. The torrential rains can be attributed to a large feed of low-level moisture pushing into the region from the Gulf of Mexico, combined with the slow movement of the storm system. “Training echoes”—successive downpours that occur over the same area due to slow storm movement in the same direction as the storm’s axis of orientation—broke out across the region. These systems tend to develop when upper-level winds are largely aligned with a low-level frontal zone. As a result, rainfall peak totals were concentrated where banding occurred.

Houston Rainfall
Houston area rainfall totals for the 24 hours before 11 a.m. CDT May 25 (Source: Harris County Flood warning System)

On the west side of Houston yesterday, rainfall rates of more than 4 inches per hour were observed, and a total of almost 11 inches was recorded in Harris County. At Houston Intercontinental Airport, the all-time record for the most rainfall in one day, set in 1946, was almost doubled on Monday as 4.34 inches were recorded.

Flood levels in many parts of the region have neared, and in some cases exceeded, record levels. At Shepherd Drive in downtown Houston this morning, the Buffalo Bayou had a maximum observed water level of 33.73 feet with a flow of 13,900 cubic feet per second. The White Oak Bayou in Houston had a maximum observed water level of 36.51 feet and a maximum observed flow of 19,500 cubic feet per second. These and several other bayous have broken their banks.

Houston Bayou
Graphical representation of flood inundation along the Buffalo Bayou in downtown Houston (Source: National Weather Service)

The Bianco River, which reaches flood stage at 13 feet, rose 26 feet within an hour and crested at more than 40 feet. The Trinity River in Dallas rose above its 30-foot flood stage to more than 40 feet on Sunday night as continued rainfall prompted the National Weather Service to extend flood advisories along the river until Thursday or Friday. Water levels in the river have fallen since late Monday morning but, with more rain forecast, may rise again.

Reported Impacts
In the Houston area, some people were stranded last night in gridlocked cars while 200 or so took refuge in a shopping mall and at the Toyota Center following an NBA playoff game. With stretches of major freeways blocked by flooding and hundreds of vehicles submerged or abandoned, travel is discouraged. METRO suspended all transportation services, and the Houston Independent School District and several other metro area districts canceled classes for today. More than 80,000 people lost power in the Houston area, and hundreds of homes were flooded. West of Houston in Wimberley, one of the worst affected areas, more than 350 homes along the Blanco River were washed away by flash floods.

In Central Texas, Shoal Creek and several other creeks overflowed their banks in downtown Austin, and the National Weather Service issued a flash flood emergency for Williamson, Travis, Hays, Caldwell, Bastrop, and Lee counties. A dam in Bastrop State Park failed on Monday evening, prompting evacuations downstream. Widespread on- and off-floodplain flooding are reported in Oklahoma and Kansas. According to KFSM-TV, homes along the Arkansas River in Van Buren, Arkansas, are threatened.

Nineteen tornadoes and at least two tornado-related deaths have been reported in Texas and Oklahoma. Reported damage from high winds includes damage to buildings in the New Orleans suburb of Kenner, the loss of a gas station canopy northeast of Jackson, Mississippi, and many reports of downed trees and utility poles and related loss of power. Hailstones 4.75 inches in diameter (larger than softballs) were reported in northeast Kansas on Monday afternoon, and golfball-size hail was noted near Nebraska City in the evening.

Outlook
The storm system is continuing to move eastward and severe storms are possible again late this afternoon and evening, particularly in north-central Texas and southern Oklahoma. Although less severe than the most recent storms, they may still deliver damaging winds (some in excess of 65 mph), large hail, more flash flooding, and possibly even tornadoes. In Southeast Texas, Flash Flood Watches continue until 6 p.m. CDT for Harris and Fort Bend counties in the Houston metropolitan area, including much of the city of Houston. AIR is continuing to monitor the situation and additional information will be made available as warranted.

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